Isaac lashing the Keys; an eyewall is building

By: Dr. Jeff Masters , 3:48 PM GMT on August 26, 2012

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Tropical Storm Isaac is steadily organizing as it lashes the Florida Keys with heavy rain and tropical storm-force winds. Sustained winds of 44 mph and 41 mph have been observed at Molasses Reef and Sombrero Key, respectively, this morning. Radar out of Key West shows an increase in spiral banding, and the beginnings of an eyewall. An Air Force Hurricane Hunter aircraft completed its first pass through the center of Isaac near 11:30 am EDT, and did not find the pressure had fallen, or that the peak winds had increased. Infrared and visible satellite loops show that Isaac is a large and increasingly well-organized storm. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is quite good and increasing to the north, but is lacking elsewhere. Moderate wind shear and dry air to the south are interfering with heavy thunderstorm development on Isaac's south side. Heavy rains from Isaac are lingering over Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least four people.


Figure 1. Morning reflectivity image from the Radar out of Key West radar shows the northwest section of an eyewall beginning to form to the southeast of the city.

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs have diverged significantly, and we can no longer be confident we know where Isaac will make landfall on the Gulf Coast. One camp of models, the UKMET and ECMWF, predict that a trough of low pressure moving across the Southeast U.S. will be strong enough to turn Isaac north to a landfall in the Florida Panhandle. The other set of models, the GFDL, GFS, and HWRF, predict the trough will bypass Isaac, and a ridge of high pressure will build in and force Isaac to a landfall over Louisiana. The official NHC forecast averages out these two extremes, calling for a landfall midway between the two solutions. Odds are, one of the two model solutions will turn out to be the correct one, and the NHC will be forced to make a substantial adjustment in their forecast track to the east or the west. Isaac has the potential to drop torrential rains capable of causing serious flooding and drought relief over the South. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over Southeast Louisiana, where it predicts Isaac will make landfall. The ECMWF model, however, these heavy rains will fall more over the Florida Panhandle, Alabama, and Georgia.


Figure 2. A hurricane forecaster's dilemma: which set of models is correct? The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs have diverged significantly. Our two top models--the GFS and ECMWF--have 72-hour forecasts that are about 350 miles apart. The ECMWF forecast is not shown here, but lies just to the west of the UKMET forecast (white line.)

Intensity forecast for Isaac
Isaac survived passage over Hispaniola and Cuba relatively intact. It's large size aided this. Isaac is over very warm waters of 31°C (88°F) with high total heat content in the Florida Straits, but is encountering moderate wind shear of 10 - 15 knots due to upper-level winds out of the southwest. This shear is predicted to relax to the light range tonight as an upper-level anticyclone becomes established over the storm. This should allow for more substantial intensification after Isaac passes the Florida Keys. However, the total heat content of the ocean decreases for Isaac Monday morning as it encounters a relatively cool ocean eddy in the Southeast Gulf of Mexico. If Isaac takes a more westerly track, passing due south of the Central Louisiana coast, the storm will encounter a modest warm eddy, which would aid intensification. The intensify forecasts from the various models are very divergent. The latest 06Z (2 am EDT) run of the GFDL model keeps Isaac as a strong tropical storm until landfall in Louisiana. Isaac will undergo rapid intensification into a Category 3 hurricane as it hits New Orleans, says the latest 06Z (2 am EDT) run of the HWRF model. The ECMWF model has Isaac as a strong Category 2 storm with a central pressure near 950 mb as it hits near the Alabama/Florida border.

Comparing Isaac with Ike of 2008
The current situation with Isaac is similar in some ways to that of Hurricane Ike of 2008. Ike spent considerable time over Cuba, weakening from a Category 4 to a Category 1 storm. The storm couldn't put its energy into building a strong inner core, but it was able to build up its outer rainbands that were over very warm waters. This resulted in a major expansion of its wind field, with tropical storm-force winds extending out 275 miles from the center at one point. Ike was able to intensify into a Category 2 storm on its path towards Texas, and had an unusually low pressure for a Cat 2 storm with 100 mph winds--944 mb. That's a central pressure more typical of a Category 3 storm, but Ike could only manage Category 2 winds, since it had such a large chunk of the atmosphere to keep spinning. With Isaac's TS winds already extending out to 205 miles, maybe we'll see another Ike-type situation as it intensifies--the storm will have an unusually low pressure in order to keep a huge wind field spinning, but never make it above Category 2, since it will take so long to spin up such a large wind field.

Storm surge forecast for Isaac
Isaac is a very large storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac's large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. The latest 3:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0 on a scale of 1 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 1 to 6. A 2008 paper by Irish et al., The influence of storm size on hurricane surge, found that large storms like Isaac are capable to delivering a 30% larger storm surge to the coast than a smaller storm with the same maximum wind speeds. The angle with which the storm hit the coast is important, too--a storm moving due north or slightly east of north will deliver a storm surge about 10% greater than a storm moving NNW or NW. Consult our Storm Surge pages for detailed information on what the risk is for the coast. I expect that Isaac's storm surge will be about 30% higher than the typical surge one would expect based on the maximum wind speeds.

Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located about 650 miles west of the Cape Verde Islands off the coast of Africa. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 50% chance of developing by Tuesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands. However, both the GFS and ECMWF models predict that a tropical wave that has not yet emerged from the coast of Africa may develop late this week, and potentially take a more westward track towards the Lesser Antilles, arriving around September 2.

Angela Fritz will have a new post here by 6 pm EDT. For the next few days, I plan to do the morning blog post, and Angela will be doing the late afternoon post. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will be on either in the afternoon or evening on Sunday, Monday, and Tuesday.

Jeff Masters

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Quoting Maineweatherguy20023:

The last time i am posting this image


It's the WRONG image. Just look at the top. 11am EDT.
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2074. air360
If the models keep pushing this more left left left - it will soon have moved all the way around the compass and be heading back S or E again <--(just a joke people...dont freak)
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Quoting Maineweatherguy20023:

S S????

Old... Get a new one before going against people. Geez...
Member Since: August 1, 2011 Posts: 28 Comments: 8075
Quoting yoboi:
levi i went to met school, if ya want to really learn weather, i ask you for one storm to only use current surface maps and radar and you will learn alot...it's old school but you will learn to see the BIG picture...just some advice ya can, can it or take it but if ya go to met school they will require ya to do it that way once or twice....


What do surface maps tell you?
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2071. LBAR
Another huge "win" for the GFS. I stand corrected!
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Quoting Maineweatherguy20023:

S S????


thats the 11am...
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Quoting wpb:
there keeping a ts for at leat another 24 hr and max out high end cat1 minor2

The last time i am posting this image
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Quoting bappit:

It hasn't strengthened much. Check out the recon and surface obs if you don't believe them.
Overall appearance is better than 12 hours ago. pressures falling. Winds will catch up soon.
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Still barely shifted....man. What in the world.
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Quoting Maineweatherguy20023:


??????


Old.
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Isaac has way too much open territory to cover before landfall.
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Quoting floridaboy14:
really? by monday afternoon he becomes a hurricane? seriously its better to overcast than downcast this storm


??????
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2062. flcanes
Quoting floridaboy14:
really? by monday afternoon he becomes a hurricane? seriously its better to overcast than downcast this storm

true
Member Since: August 20, 2012 Posts: 13 Comments: 1254
really? by monday afternoon he becomes a hurricane? seriously its better to overcast than downcast this storm
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2060. wpb
there keeping a ts for at leat another 24 hr and max out high end cat1 minor2
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Quoting NICycloneChaser:


He's put the pressure at 994, when recon clearly shows it to be 992.


No, wait, either it's been changed or iI'm going mad. Either is entirely possible.
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Quoting MississippiWx:


You mean the ensembles.

12z Euro Ensembles are in SE Louisiana/MS.



Thanks
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2057. bappit
Quoting floridaboy14:
TWC channel saying isaac hasnt stregthened much. thoughts?

It hasn't strengthened much. Check out the recon and surface obs if you don't believe them.
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...CENTER OF ISAAC PASSING JUST SOUTH OF KEY WEST... ...U.S. Warnings in Effect...
5:00 PM EDT Sun Aug 26
Location: 24.2°N 82.3°W
Moving: WNW at 16 mph
Min pressure: 992 mb
Max sustained: 60 mph
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Quoting Bluestorm5:
Here come the bashing...


S S????
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Quoting muddertracker:


The L.A. governor is pretty impressive. Just said he's not leaving his state while the citizens are in harms way. Way cool.


He is an EXCELLENT Governor!!! He takes good care of his State!!!!
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Looks like a Katrina Path.
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Pasch nurfed the intensity forecast. No hurricane until Tuesday? Really Pasch?
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Quoting Jedkins01:


Moistening? There is no "dry slot" in a well formed tropical system like this. Just because there isn't rain in a region of a storm, doesn't mean there isn't high moisture there, that's a big meteorological no-no.

Here you go, tell me where in Isaac can you find a dry slot below? This is about as much moisture as the air can possibly hold.



I agree...PWats less than that cause flooding rains on systems onland...even when it's the season for that type of moisture.

I concur.
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As will the DNC.
Quoting brywalker:


Maybe, but the rest of the RNC will go on as scheduled while people are suffering or dying because of Isaac

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Ap is reporting Oil and Gas are evacuating platforms in the Gulf... No surprise here back to what you were doing...
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That is an awful intensity forecast.
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wtf is up with the dry air? in the past 5 years i have seen more dry air in the tropics than i ever have in my life.....

how in the hell there is so much dry air in the tropics is beyond me??? could it be a result of global warming?
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Quoting TropicalAnalystwx13:
Pasch doesn't have Isaac becoming a hurricane until 28/06Z (Tuesday morning). He has officially lost his mind.


He's put the pressure at 994, when recon clearly shows it to be 992.
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Quoting Jedkins01:


Moistening? There is no "dry slot" in a well formed tropical system like this. Just because there isn't rain in a region of a storm, doesn't mean there isn't high moisture there, that's a big meteorological no-no.

Here you go, tell me where in Isaac can you find a dry slot below? This is about as much moisture as the air can possibly hold.

to the SW there a little.
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Not to us. Gotta makes some calls. :)
Member Since: August 24, 2010 Posts: 0 Comments: 254
The only thing that is inhibiting isaac to a RI is the dry air in the south and southwest...THE ONLY!!!
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Here come the bashing...

Member Since: August 1, 2011 Posts: 28 Comments: 8075
Hurricane warning issued for Gulf coast from Morgan City, LA, to Destin, FL.
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Pasch doesn't have Isaac becoming a hurricane until 28/06Z (Tuesday morning). He has officially lost his mind.
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Quoting washingtonian115:
MSWX how far do you live away from the coast?.


About 50 miles. Far enough inland to not worry about water, of course, but winds/tornadoes/flooding from heavy rain is a different story.
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Quoting Jedkins01:


Moistening? There is no "dry slot" in a well formed tropical system like this. Just because there isn't rain in a region of a storm, doesn't mean there isn't high moisture there, that's a big meteorological no-no.

Here you go, tell me where in Isaac can you find a dry slot below? This is about as much moisture as the air can possibly hold.


You didnt get the suspect area on ur map :)
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2037. GetReal
Well just spotted some lines forming at gasoline stations... The price of gasoline has risen $.09, since I filled my gasoline containers this morning.
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Quoting 1900hurricane:
Differences in the big four: @72 hours:



@96 hours:



GFS consistency i see there. I would expect the NHC to shift slightly more west at the next advisory.
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weatherbellmodels has Isaac going west to New Orleans.
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2034. yoboi
levi i went to met school, if ya want to really learn weather, i ask you for one storm to only use current surface maps and radar and you will learn alot...it's old school but you will learn to see the BIG picture...just some advice ya can, can it or take it but if ya go to met school they will require ya to do it that way once or twice....
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2032. Axelia
Kctinney, regarding Orlando area, I'm an hour west and all we've had thus far is one strong rain/wind band earlier today. Since it's moving so far to the west, I'm not expecting that much of an event in this area. Just some more rain and wind but not as fierce as was projected 24 hours ago.
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Quoting MississippiWx:


You mean the ensembles.

12z Euro Ensembles are in SE Louisiana/MS.



12z operational still shows a fl/al landfall
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Quoting ProgressivePulse:


Hopefully unless it works out that dry slot. Those discreet cells that are starting to form there are an indication of moistening.




Moistening? There is no "dry slot" in a well formed tropical system like this. Just because there isn't rain in a region of a storm, doesn't mean there isn't high moisture there, that's a big meteorological no-no.

Here you go, tell me where in Isaac can you find a dry slot below? This is about as much moisture as the air can possibly hold.

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2029. airmet3
The HGX NWS discussion says the NHC is going with Mississippi at this time, though us Texans need to keep an eye on it for sure.
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MSWX how far do you live away from the coast?.
Member Since: August 14, 2010 Posts: 10 Comments: 17839
Quoting yoboi:


look at the latest gfs....i think it picked all this up...reed levi drake hope ya'll study the layers now might help ya'll down the road...
Quoting yoboi:



levi watch the next 24 hrs with layers ya will learn alot...not bashing ya just watch...
Let's see your marvelously insightful, educated - and what's more, flawless - analysis of the system... not bashin' ya, just sayin'.
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TROPICAL STORM ISAAC FORECAST/ADVISORY NUMBER 23
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
2100 UTC SUN AUG 26 2012

CHANGES IN WATCHES AND WARNINGS WITH THIS ADVISORY...

THE GOVERNMENT OF THE BAHAMAS HAS DISCONTINUED THE TROPICAL STORM
WARNING FOR THE NORTHWESTERN BAHAMAS.

THE GOVERNMENT OF CUBA HAS DISCONTINUED ALL WATCHES AND WARNINGS.

THE TROPICAL STORM WATCH FROM NORTH OF SEBASTIAN INLET TO FLAGLER
BEACH IS DISCONTINUED.

THE HURRICANE WARNING IS CHANGED TO A TROPICAL STORM WARNING FOR THE
FLORIDA KEYS INCLUDING THE DRY TORTUGAS AND FLORIDA BAY.

THE HURRICANE WARNING FOR THE WEST COAST OF FLORIDA FROM BONITA
BEACH SOUTHWARD TO OCEAN REEF IS CHANGED TO A TROPICAL STORM
WARNING.

THE HURRICANE WATCH IS CHANGED TO A HURRICANE WARNING FOR THE
NORTHERN GULF OF MEXICO COAST FROM EAST OF MORGAN CITY LOUSIANA TO
DESTIN FLORIDA...INCLUDING METROPOLITAN NEW ORLEANS...LAKE
PONTCHARTRAIN...AND LAKE MAUREPAS.

THE TROPICAL STORM WARNING FROM SUWANNEE RIVER TO ANCLOTE KEY IS
DISCONTINUED.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A HURRICANE WARNING IS IN EFFECT FOR...
* EAST OF MORGAN CITY LOUISIANA TO DESTIN FLORIDA...INCLUDING
METROPOLITAN NEW ORLEANS...LAKE PONTCHARTRAIN...AND LAKE MAUREPAS

A HURRICANE WATCH IS IN EFFECT FOR...
* EAST OF DESTIN TO INDIAN PASS FLORIDA

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE FLORIDA PENINSULA FROM SEBASTIAN INLET SOUTHWARD ON THE EAST
COAST AND FROM ANCLOTE KEY SOUTHWARD ON THE WEST COAST.
* FLORIDA KEYS...INCLUDING THE DRY TORTUGAS AND FLORIDA BAY
* LAKE OKEECHOBEE

A HURRICANE WARNING MEANS THAT HURRICANE CONDITIONS ARE EXPECTED
SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN THE NEXT 24
HOURS. PREPARATIONS TO PROTECT LIFE AND PROPERTY SHOULD BE RUSHED
TO COMPLETION.

A HURRICANE WATCH MEANS THAT HURRICANE CONDITIONS ARE POSSIBLE
WITHIN THE WATCH AREA. A WATCH IS TYPICALLY ISSUED 48 HOURS
BEFORE THE ANTICIPATED FIRST OCCURRENCE OF TROPICAL-STORM-FORCE
WINDS...CONDITIONS THAT MAKE OUTSIDE PREPARATIONS DIFFICULT OR
DANGEROUS.

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

TROPICAL STORM CENTER LOCATED NEAR 24.2N 82.3W AT 26/2100Z
POSITION ACCURATE WITHIN 25 NM

PRESENT MOVEMENT TOWARD THE WEST-NORTHWEST OR 295 DEGREES AT 14 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 994 MB
MAX SUSTAINED WINDS 50 KT WITH GUSTS TO 60 KT.
50 KT....... 60NE 0SE 0SW 60NW.
34 KT.......180NE 120SE 80SW 180NW.
12 FT SEAS..150NE 150SE 120SW 120NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.

REPEAT...CENTER LOCATED NEAR 24.2N 82.3W AT 26/2100Z
AT 26/1800Z CENTER WAS LOCATED NEAR 23.9N 81.5W

FORECAST VALID 27/0600Z 25.1N 83.9W
MAX WIND 55 KT...GUSTS 65 KT.
50 KT... 60NE 0SE 0SW 60NW.
34 KT...180NE 120SE 80SW 180NW.

FORECAST VALID 27/1800Z 26.3N 85.9W
MAX WIND 60 KT...GUSTS 75 KT.
50 KT... 60NE 0SE 0SW 60NW.
34 KT...180NE 120SE 80SW 180NW.

FORECAST VALID 28/0600Z 27.7N 87.6W
MAX WIND 70 KT...GUSTS 85 KT.
64 KT... 30NE 20SE 10SW 20NW.
50 KT... 60NE 60SE 40SW 50NW.
34 KT...180NE 150SE 80SW 160NW.

FORECAST VALID 28/1800Z 28.9N 88.5W
MAX WIND 80 KT...GUSTS 100 KT.
50 KT... 70NE 60SE 50SW 60NW.
34 KT...180NE 150SE 90SW 160NW.

FORECAST VALID 29/1800Z 30.7N 89.5W
MAX WIND 85 KT...GUSTS 105 KT.
50 KT... 60NE 70SE 60SW 40NW.
34 KT...100NE 150SE 100SW 90NW.

EXTENDED OUTLOOK. NOTE...ERRORS FOR TRACK HAVE AVERAGED NEAR 175 NM
ON DAY 4 AND 225 NM ON DAY 5...AND FOR INTENSITY NEAR 20 KT EACH DAY

OUTLOOK VALID 30/1800Z 32.5N 90.0W...INLAND
MAX WIND 30 KT...GUSTS 40 KT.

OUTLOOK VALID 31/1800Z 34.5N 90.0W...INLAND
MAX WIND 20 KT...GUSTS 30 KT.

REQUEST FOR 3 HOURLY SHIP REPORTS WITHIN 300 MILES OF 24.2N 82.3W

NEXT ADVISORY AT 27/0300Z

$$
FORECASTER PASCH
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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